Funds are requested to support the 3rd international workshop to determine the status of the genetic and physical maps of human chromosome 21 (HC21). (The first workshop was held in Bethesda, MD and organized by Dr. D. Cox; the 2nd workshop was held in Denver, CO and was organized by Dr. D. Patterson.) The workshop entitled "Mapping Chromosome 21: Present Status and Future Strategies" will be held in Baltimore in April 1992 for 2 days and will include 46 scientists from the United States and 26 scientists from abroad. The primary focus of the meeting will be to assess progress on the physical and genetic map of HC21 since the 1991 workshop and the status of cDNA mapping on HC21. An important aspect will be to assess the progress of the Joint YAC Screening Effort which was established at the 1990 workshop and to make any adjustments felt necessary in that screening effort. An attempt will be made to determine the feasibility and desirability of attempting to completely close the physical map of HC21. Assessment will be made of the progress toward a high resolution genetic linkage map using all available published and unpublished data so that strategies can be devised to enhance completion of this objective, if necessary. The conference will assess the status of the comparative mouse map of regions of the mouse genome homologous to HC21, assess available data on isolation of gene sequences from HC21, and determine the advisability of undertaking a major effort toward isolating a large number of HC21 specific gene sequences.